1. Field of the Invention
The present invention relates to chip containing electronic devices and, in particular, to Multichip Ceramic Modules (MCM's) containing a plurality of chips electrically connected to a substrate with solder bumps, which chips are underfilled with a reworkable composition which allows one or more chips to be removed from the module and replaced.
2. Description of Related Art
Electronic components such as Multichip Ceramic Modules (MCM's) are key components for many high-end computer servers and mainframes. MCM's are particularly important because they contain numerous chips on a substrate. However, if one of the chips is defective due to electric issues it must be replaced. Without the ability to replace the chip, typically termed rework of the chip, the cost of MCMs would become prohibitive.
Controlled Collapse Chip Connection is an interconnect technology developed by IBM as an alternative to wire bonding. This technology is generally known as C4 technology or flip chip packaging. Broadly stated, one or more integrated circuit chips are mounted above a single or multi-layer substrate and pads on each chip are electrically connected to corresponding pads on the substrate by a plurality of electrical connections such as solder bumps. The integrated circuit chips may be assembled on the substrate in a solder bump array such as a 10×10 array. The chip bumped substrate is then typically electrically connected to another electronic device such as a circuit board by pin connectors with the total package being used in an electronic device such as a computer.
Flip chip packaging is described in U.S. Pat. No. 5,191,404 which patent is hereby incorporated by reference. In general, flip chip joining is desirable for many applications because the footprint or area required to bond the chip to the substrate is equal to the area of the chip itself. Flip chip joining also exploits the use of a relatively small solder bump which typically measures a height of approximately 1 mil to 1.5 mils and a width of approximately 2 to 4 mils to join the pads on the chip to corresponding pads on the substrate. Electrical and mechanical interconnects are formed simultaneously by reflowing the bumps at an elevated temperature. The C4 joining process is self-aligning in that the wetting action of the solder will align the chip's bump pattern to the corresponding substrate pads. This action compensates for chip to substrate misalignment up to several mils which may be incurred during chip placement.
In the joined flip chip package there is necessarily an opening or space between the pad containing surface of the integrated circuit chip and the pad containing surface of the joined substrate resulting from the thickness of the pads on each surface and the solder bump connection between the pads. This open space cannot be tolerated because any interference with the solder connections will adversely affect the performance of the package. For example, moisture, mobile ions, acidic or alkaline species, which could be from corrosive surroundings or from infusion of thermal paste used to increase heat transfer from the chip, and the mechanical integrity of the chip due to the possible breaking of the solder bump electrical connections are all serious problems. To solve these problems, the solder bumps of the joined integrated circuit chips and substrate are typically encapsulated totally with various types of material, such as a liquid, and such a sealant is used around the chip edges to seal the joined opening.
Flip chip bonding offers many advantages in electronics manufacture and one of the most important is the ability to remove and replace the chip without scrapping the substrate and adjacent chips. This removal of the chip by heating and lifting of the chip from the substrate and replacement with typically a new chip is termed rework and can be performed numerous times without degrading the quality or reliability of the reworked electronic component.
Encapsulation of the flip chip packages however presents rework and other problems. The flip chip package must be reliable and thermo mechanical mismatches between the encapsulant, chip, substrate and/or solder bumps must be minimized to avoid stressing and damaging of the package, in particular, the solder interconnects. The encapsulant must also be able to be heated and softened or preferably be soluble in a solvent such as xylene for the lift-off (rework) procedure.
The conventional underfill process to encapsulate the space between a single chip bonded to a substrate surface typically applies the underfill material to the substrate adjacent to the periphery of the chip to be underfilled. Capillary action draws the underfill encapsulation material into the space between the chip and the substrate to form a void free filled space between the chip and the substrate.
Historically, there are two types of MCM seals to protect the chips from damage, namely, a hermetic seal and a non-hermetic seal for preventing moisture permeation and mobile ion ingress to the chip joints. Most hermetic seals are formed according to metal isothermal interdiffusion mechanism, such as C-ring seal. Non-hermetic seals also named as reliability without hermeticity (RWOH), use polymer based composite material to form a non-hermetic seal band. Compared with hermetic seal, non-hermetic seal is less protective in terms of moisture ingress and mobile ion permeation. Therefore, non-hermetic sealed MCMs show varied level C4 corrosion when exposed to temperature and humidity environment during real application.
Regardless of the type seal, however, it is necessary to underfill the chips and an ideal reworkable underfill (RUF) should possess several key characteristics such as-reworkability, low modulus, thermal stability, no interference with surrounding materials, such as thermal paste, compatible with sealing band or c-ring which is used for sealing MCM's, and be environmentally safe. Also, RUFs should be reworkable in neutral solution to prevent any chemical attack to C4 interconnects; have low deep thermal cycling introduced thermomechanical stress; be thermally and chemically stable at 125° C. for 1,000 hours; and provide sufficient protection against moisture, CO2 ingress and carboxylic acid permeation.